1. Field of the Invention
The invention relates to percussive machinery used to comminuting brittle materials and penetrating into brittle materials. Preferred applications of the invention are deep drilling operations for the exploitation of oil- and gas wells, geothermal energy sources and generally for reconnaissance drilling into deep rock formations.
Further applications are for example the driving of tunnels and shafts and demolition work in environments without direct electric power supply. Furthermore, the invention can be used for percussive drilling and demolition with hand-driven tools.
2. Description of the Prior Art and Related Information
For drilling operations to depth of several thousand meters, the rotary drilling method is by far the most commonly used technique. This method is very suitable for the drilling in soft and semihard rock formations. The achievable drilling rate is however significantly decreased, if hard (crystalline) rock formations are encountered.
It is known for a long time that percussive drilling is much more suitable for crystalline hard rock, than with roller cone bits or rotating polycrystalline diamond compact (PDC) bits, whose mode of action is based on quasistatic uniaxial loading and shear, respectively. For example, the drilling rate of percussive machinery was found to be 10 times higher in granite than with roller cone bits. Further advantages of percussive drilling are low static loads (weight on bit, WOB) as well as a higher stability of the drilling process with respect to off-axis deviations.
Utilization of percussive drilling is state of the art in near surface drilling operations for a long time, for example for the excavation of blast holes in open-cast mining or for the near surface geothermics in hard rock formations.
For these purposes, a large number of apparatus and methods are described.
With respect to the location of the driving mechanism within the drill string, percussion drills can essentially be divided into two groups:
Top hammers (surface-operating) and down the hole (DTH) hammers. The former are mounted on a drill rig that remains above surface during drilling operation. The percussive action is transmitted in the form of longitudinal elastic waves through a stiff drill string. Due to the attenuation of these waves, the depth achievable with this method is usually restricted to less than 100 meters.
For deeper drilling depths DTH hammer are the only viable method. Here, the percussive mechanism is located directly behind the drillbit and is lowered town into the borehole together with the drill string. The energy required to drive the percussive mechanism is traditionally provided by pressurized air or water. However, a system purely based on pressurized air without drilling fluid would be problematic concerning the removal of the cuttings from the bottom of a deep borehole. A system based on a combination of surface-supplied pressurized air or gas as energy source for percussion and a thixotropic drilling fluid for the removal of the cuttings would require ever stronger compressors to overcome the quickly rising pressure at the borehole bottom—moreover, serious problems with the severalfold volume increase of the expanding gas on the way back to the surface would be encountered.
Conventional hydraulic percussion drills function via acceleration and deceleration of the water column inside the borehole. The abrupt stopping of the downward flow causes an impulse that is transmitted to the drillhead. As the inertia of the water column of the borehole increases linearly with drilling depth, maintaining the same percussive frequency would afford an ever increasing energy input. This requirement causes the energetic efficiency of this technique to become prohibitively low for large depths.
Moreover, percussive mechanisms that operate via direct throughput of drilling fluid in this or a similar manner are apt to extensive wear caused by the abrasive action of solid particles that are suspended within the fluid.
EP 0096 639 A1 presents a DTH-drill that is operating according to the principle of an internal combustion engine. Compressed air is alternatingly forced into an upper and a lower part of a cylinder chamber. Additionally, gasoline fuel is injected into the upper chamber. The fuel-air mixture ignites and the additional combustion pressure drives a striker piston towards an anvil. Exhaust gases and cooling air are to be transported back to the surface by appropriate ducts.
A similarly operating internal combustion hammer is described in DE 39 35 252 A1. It is comprised of a housing with concentric rows of multiple drill rods that are terminated by impact teeth at its lower end facing the rock to be drilled. The rods with the attached impact teeth are driven by combustion cylinders inside the apparatus that are sequentially fired to impact the rock. The device requires a number of supply pipes that carry pressurized air and fuel towards and exhaust gases from down-the-hole apparatus to the surface. Also electric cables are required for ignition and valve operation of the combustion chambers.
WO 2001/040 622 A1 discloses a device for generating pressure pulses in a borehole on the basis of a combustion heat engine which. The downhole pulser has a housing which accommodates a cylinder and a spring-loaded piston which are being arranged in that manner as to perform a combustion stroke of a combustible gas mixture. The combustion stroke causes a hammer being attached to the piston to impact an anvil. The components are reverted into their initial position by the means of springs. The combustion engine is supplied with hydrogen fuel and oxygen from two separate tanks. The intake of the combustion gases and exhaust of the resulting water steam is controlled by valves.
Further precussive drill bit drives on the basis of internal combustion engines are disclosed in SE 153256 C and GB 1350646 A.
DE 27 26 729 A1 and DE 30 29 710 A1 present a deep drilling device that is creating percussive pulses and is simultaneously set into a rotary motion by means of explosives or combustible gases.
All heat engines in the above-noted disclosures are operating without crank and crankshaft, as the expanding gas is acting directly on a percussive mechanism.
However, their required supply of gaseous or liquid fuel and oxidizers or explosives as well as the removal of the exhaust gases are very difficult to realize at large depths, as is the case for an electric powerline.
In deep drilling applications, in order to maintain the stability of the borehole, drilling fluids with a high gravity between 1.2 to 1.6 g/cm3 are being employed.
The hydrostatic pressure at the bottom of a liquid column of depth h is increasing by ρ·g·h with g being the gravitational acceleration and ρ may be assumed as being approximately constant. Consequently, at large depths of several 1000 m high hydrostatic pressures of several hundred to more than 1000 bar can occur.
The operation of a heat engine at an internal pressure significantly lower than the hydrostatic pressure can be hardly imagined as in the most cases the percussive mechanism would also have to overcome this pressure difference. Moreover, the cylinder and other parts of the machine may be compressed or even collapse.
Conversely, pre-compression of the gaseous working of the engine at the surface can pose the risk of explosion.
This problem may be solved by a successive pressurization of the engine during the drilling operation or lowering of the drill string which may be accomplished by a pressure line from the surface or a pressure tank being integrated into the drill string. In deep wells >4000 m and/or heat engines with a large internal working space both solutions receive further restrictions.
A pressure tank pre-compressed to the full terminal pressure would be almost as hazardous as a similarly pressurized heat engine itself. Without pressurization, the required initial volume (i.e. the length of a compensation tank) might become unacceptably large with respect to the typical diameter of a drill string, as the Boyle-Mariotte law p1·V1=p2·V2 applies.